Printing of photographic colour negatives

ABSTRACT

Apparatus for screening photographic colour negatives to determine the exposure time and colour filtering required to produce an acceptable colour print. The apparatus incorporates a photoresistor connected in series with an adjustable resistance calibrated in terms of exposure times, the photoresistor and adjustable resistance constituting two arms of a bridge circuit. A further two arms are constituted selectably by either a fixed resistor network or by one or other of two potentiometers calibrated in terms of colour filter densities, switches being provided to enable selection. Alternatively one potentiometer appropriately switched can serve the function of the two potentiometers. A detector circuit senses balance in the bridge circuit. The calibration of the adjustable resistance and potentiometers, and the way the apparatus is used are explained.

United States Patent [191 Aston 1 1 PRINTING OF PHOTOGRAPHIC COLOUR NEGATIVES [76] Inventor: Keith Aston, 20 Summerville Gardens, Surrey, England [22] Filed: Oct. 27, 1971 [21] Appl. No.: 192,956

Primary Examiner-Robert P. Greiner Att0rneyl-Iarry W. F. Glemser et al.

[ 51 Dec. 11, 1973 [57] ABSTRACT Apparatus for screening photographic colour negatives to determine the exposure time and colour filtering required to produce an acceptable colour print. The apparatus incorporates a photoresistor connected in series with an adjustable resistance calibrated in terms of exposure times, the photoresistor and adjustable resistance constituting two arms of a bridge circuit. A further two arms are constituted selectably by either a fixed resistor network or by one or other of two potentiometers calibrated in terms of colour filter densities, switches being provided to enable selection. Alternatively one potentiometer appropriately switched can serve the function of the two potentiometers. A detector circuit senses balance in the bridge circuit. The calibration of the adjustable resistance and potentiometers, and the way the apparatus is used are explained.

10 Claims, 6 Drawing Figures f/A/[f I 5 747 i PATENTEUUECI 1 1m SHEET 2 BF 5 PATENTEI] DEC! 1 I975 sumsnrg PRINTING OF PHOTOGRAPHIC COLOUR NEGATIVES This invention concerns improvements in or relating to the printing of photographic colour negatives.

One well known process for printing a batch of different colour negatives using a non-automatic colour printer, is to select from the batch one negative whose colour content is representative (in so far as is possible) of the colour content of all the negatives of the batch, to adjust by a process of trial and error the colour composition of the printing light beam of the colour printer by inserting different density colour correction filters into the light beam so that the printer when adjusted will produce an acceptable colour print of the selected negative within a set expoure time, and to print all of the negatives in the batch with the colour printer so adjusted. The shortcomings of such a process are readily apparent since by this process the majority of negatives will be printed with light of a colour composition which is not suited to the colour balance of the negative.

The alternative process of printing every negative of a batch individually, with each negative being printed several times by a trial and error process in which the colour composition of the printing light is adjusted for each print until an acceptable print is obtained, generally is too time consuming and costly a process in regard to the materials required to be entertained by anyone other than the professional photographer interested in producing quality prints. Neither process is convenient for the amateur enthusiast wishing to print his own colour negatives and consistently obtain good results.

It is the object of this invention to provide a means to enable the amateur enthusiast to print negatives individually without being obliged to perform repeated trial and error prints in order to realize acceptable results.

According to this invention therefore there is provided apparatus for screening photographic colour negatives to determine their colour balance including a photoconductive or photoemissive cell, an adjustable resistance connected in series with the current path of the said cell to constitute with the cell a potential divider, a potentiometer connected to enable selection of a potential for comparison with the potential at the junction of the cell and the adjustable resistance, and a detector circuit arranged to indicate when the potential at said junction bears a predetermined relation to (e.g. is equal to) the potential selected by the potentiometer, the arrangement furthermore being such as to enable the adjustable resistance to be set to a value bearing a predetermined relation to (e.g. equal to) the impedance presented by the photoconductive or photoemissive cell to current flow therethrough.

Although many types of photoconductive or photoemissive cell could be used in the apparatus defined above, for example a photoconductive diode or transistor or a photoemissive vacuum tube could be used, for reasons of cheapness and simplicity in circuit design it is preferred to employ a photoresistor as the photoconductive cell in the apparatus. In the following, the invention will be explained in relation to an exemplary apparatus employing a photoresistor; it will be appreciated that the invention is not limited to such an apparatus.

it has been found that by use of the above-defined apparatus in the manner detailed immediately hereafter, having once calibrated the apparatus and determined the printing requirements necessary to produce an acceptable print from a selected standard" negative, the requirements for obtaining acceptable prints from other negatives can readily be obtained.

Consider for purposes of illustration the use of the apparatus defined above in the printing of a conventional photographic colour negative using printing paper which is sensitive to light of the three primary colours green, blue and red by means of the printing process in which colour correction filters (respectively magenta, yellow and cyan) of appropriate densities having regard to the colour composition of the negative to be printed are inserted into the printing light beam and a single time exposure of the negative is made with the colour corrected light passed by the filters.

It is first necessary to calibrate the apparatus which requires an average standard negative (i.e. a negative composed of substantially equal amounts of the complementary colours magenta, yellow and cyan) to be selected and printed by a process of repeated trial and error until for a particular exposure time the densities of the magenta, yellow and cyan colour correction filters required to be inserted in the printing light beam to obtain an acceptable print of the negative for that exposure time have been determined. With the printer set to operate in accordance with the set of printing conditions determined as above and with the standard negative in the negative carrier of the printer, the printing light, after passing through the colour correction filters and the negative, is caused to impinge upon the photoresistor of the apparatus according to the invention, the photoresistor being disposed at such a point in the printing lightbeam as to receive a representative proportion of the light in the beam or alternatively an optical system being provided to focus all the light in the printing light beam upon the photoresistor. A primary tricolour filter, complementary in colour to one of the colour correction filters inserted into the printing light beam in determining optimum printing conditions for the standard negative (preferably a green colour filter which is complementary to the magenta colour correction filter), is placed across the photoresistor and the said adjustable resistance is adjusted to a setting where its resistance equals that of the photoresistor; this setting of the adjustable resistance is noted. The green colour filter is then removed from in front of the photoresistor and a blue colour filter is put in its place; leaving the setting of the adjustable resistance unchanged, the potentiometer is adjusted until the detector circuit provides its indication and the potentiometer setting is noted. This last step is then repeated with a red colour filter in place of the blue filter across the photoresistor and the potentiometer setting again noted. The above three steps are then repeated to determine a further set of adjustable resistance and potentiometer settings, each step being carried out with a neutral density filter inserted into the printing light beam such as to reduce by half the intensity of light incident upon the photoresistor and the setting of the respective adjustable resistance or potentiometer being restored to its first-determined setting (i.e., its setting as determined with full illumination of the photoresistor) at the completion of each step and before proceeding to the next step. In this way two sets of adjustable resistance and potentiometer settings are noted. To calibrate the apparatus, the first-determined adjustable resistance setting is first labelled with the exposure time of the set of printing conditions to which it corresponds and the second-determined setting is labelled with twice that exposure time since half as much light will require double the exposure time, and a logarithmic scale is drawn up to fit the two exposure time settings. Next, the respective first-determined potentiometer settings for the blue and red filters are labelled with the respective yellow and cyan filter densities of the set of printing conditions determined by trial and error to which they correspond, and the respective seconddetermined settings are labelled with filter densities 3db down from the determined trial and error filter densities since filter densities are logarithmically related to their light transmittance, and separate linear scales are drawn up to fit the yellow and cyan filter densities respectively.

To facilitate use of the apparatus, the scales are arranged to be movable relative to their respective adjusting control knobs. The purpose of this will become clear from consideration of the following description of the way the apparatus is to be used.

As an alterative to calibrating the adjustable resistance with a logarithimic scale, the adjustable resistance may be arranged to provide logarithimic resistance variations for equal movements of a control knob or slide so that the knob or slide can be calibrated linearly. Having once been calibrated, the apparatus is ready for use. It will be appreciated that calibration of the apparatus would generally be effected by the manufacturer, so that an apparatus purchased from a retailer would be already calibrated.

Starting from the assumption that an already calibrated apparatus is available, a batch of different photographic colour negatives would be treated by selecting from the batch an average negative whose colour composition is fairly representative (in so far as is possible) of the colour compositions of all the negatives in the batch, and printing such an average negative by a process of repeated trial and error until the printing conditions required to produce an acceptable print from that negative are determined. As an alternative to selecting an average negative from a batch to be printed, a standard negative may be provided by the manufacturer. The apparatus according to the invention is then disposed so that with the printer adjusted in accordance with the determinedconditions for printing the selected average negative and with the average negative in the negative holder of the printer, the printing light falls upon the photoresistor. A green filter is then inserted across the photoresistor and the adjustable resistance set to a setting where an indication is provided that its resistance is equal to that of the photoresistor. This setting of the adjustable resistance is made to correspond to the determined exposure time for printing the average negative by moving the scale of the adjustable resistance relative to the control knob by which the resistance is adjusted. A blue filter is then inserted in front of the photoresistor and the green filter removed, and the potentiometer adjusted to obtain the indication from the detector circuit; this setting of the potentiometer is made to correspond to the density of the yellow colour correction filter employed to print the average negative by adjusting the respective scale relative to the control knob or slide of the potentiometer. This last step is then repeated with a red filter in front of the photoresistor and the cyan colour correction filter density scale adjusted so that the potentiometer setting corresponds to the cyan filter density used for printing the average negative. Having thus set the apparatus for the average negative, printing conditions for any other negative of the batch are determined simply by substituting the new negative for the average negative in the negative holder of the printer with the printing light still directed upon the photoresistor, placing the green filter across the front of the photoresistor and adjusting the adjustable resistance to set its value equal to that of the photoresistor, the setting of the adjustable resistor then indicating the exposure time required for printing the new negative, then substituting a blue filter for the green filter and adjusting the potentiometer to obtain the detector circuit indication, the setting of the potentiometer against the yellow colour correction filter density scale indicating the yellow filter density required for printing the new negative, and finally repeating this last step with a red filter in place of the blue filter and taking the scale reading on the cyan filter scale to obtain the cyan filter density required for printing the new negative. The new exposure time, yellow filter density and cyan filter density thus determined are then set in the printer and printing of the new negative can proceed.

In the event that the indicated yellow and/or cyan colour correction filter density settings required for printing the new negative are less than zero density, this indicates that the amount of blue and/or red light respectively from the printer with no yellow and/or cyan filtration is insufficient to print the negative in the exposure time indicated. If this should occur, it is necessary only to increase the density of the magenta colour correction filter in the light beam and repeat the screening process detailed above on the new negative, the magenta filtration being increased until both the yellow and cyan filter densities indicated are positive.

It will be appreciated that rather than having a single adjustable potentiometer provided with two separate scales (ie one corresponding to yellow filter densities and the other to cyan filter densities), it may be preferable, for the sake of avoiding confusion of one scale with the other, to provide two separate adjustable potentiometers each an individual scale and to provide switch means for'connecting one or other of the two potentiometers in circuit with the detector. The same switch means can also be arranged for switching the photoresistor, adjustable resistor and detector circuit into a bridge circuit for enabling an indication to be provided of when the resistance value of the adjustable resistance is equal to that of the photoresistor. Such a construction can be made particularly convenient by mechanically linking the switch means with a movable filter carrier in which green, blue and red colour filters are mounted, and arranging that movement of the switch means control knob to switch appropriate components into circuit with the detector during screening of a negative to be printed also effects movement of the appropriate colour filter into the light path to the photoresistor.

In order that the invention might be understood more clearly two embodiments thereof will now be described with reference to the accompanying drawings in which:

FIG. 1 shows a schematic circuit diagram of a first embodiment to be described;

FIG. 2 is a cut away perspective view of the embodiment of FIG. 1 showing constructional details thereof;

FIG. 3 is an enlarged showing of a detail of the embodiment of FIG. 1;

FIG. 4 shows a schematic circuit diagram of a second embodiment to be described;

FIG. 5 shows a top plan view of the second embodiment;

FIG. 6 illustrates the construction of the lens unit which is shown in FIG. 5 connected by cable to the main analyser unit.

As shown in FIG. 1, the apparatus includes a Cadmium Sulphide photoresistor l which is contained within a housing together with the other components of the apparatus as shown in FIG. 2, the housing being provided with a window permitting light to enter the housing and be incident upon the photoresistor, but which alternatively could be contained in a separate head connected with the other components of the apparatus by electric leads and adapted to be coupled to the objective lens of a colour printer. Green, blue and red colour filters are mounted in a slide member (see FIG. 2) which is movable to place any one of the three filters across the light-receptive surface of the photoresistor 1.

An adjustable resistance 2 is connected to the photoresistor l and fixed resistances 3, 4, 5 and 6 complete a bridge circuit for enabling the situation where the adjustable resistance 2 has an ohmic value equal to that of the photoresistor 1 to be detected. The bridge circuit has the photoresistor l in one arm, the variable resistor 2 in another arm and resistances 3 and 4 and resistances 5 and 6 in the other two arms which have equal ohmic values. The adjustable resistance 2 is a rotary switch arranged to provide logarithimic changes in resistance for equal rotational movements of the switch adjusting knob. The resistances in the rotary switch are such that five rotational movements in any part of the switch range doubles or halves the resistance value. Furthermore, the overall ohmic value of the switch is high, about 20 megohms.

A detector circuit consisting of field effect transistor T1, transistors T2 and T3 connected in a high gain amplifier configuration, indicator lamp l and various biassing resistors is coupled directly to the junction of the photoresistor 1 and adjustable resistance 2 and in connectable through the intermediacy of a selector switch 9 to the diagonally opposite point of the bridge'circuit. That is to say, the gate electrode of field effect transistor (FET) T1 is connected to the junction of photoresistor l and adjustable resistance 2, and the source electrode of FETTl is connectable through the selector switch 9 to the junction of resistors 4 and where, by virtue of resistors 3 and 6 being of equal value and resistors 4 and 5 being of equal value, the potential is half that of the supply.

Ideally the detector circuit should operate when the potential of the gate electrode of FET Tl becomes equal to that of its source electrode: in practice, however, there is an ofi'set voltage between the gate and source electrodes of FET T1 which is constant in value. To compensate this unavoidable effect, a diode D1 is inserted in the source lead of FET T1 to introduce an approximately equal and constant voltage drop into the line from the selector switch to the source electrode of FET Tl. By this means, the detector circuit is not much affected by fall-off in battery supply voltage as the battery ages.

A capacitor 11 is connected between the gate electrode of FET T1 and the negative battery terminal to smooth out, at least to some extent, the variation in the signal applied to the gate electrode resulting from lOOl-l flicker in the light from a printer light source powered from the 50H, mains supply. This enables the balance point of the bridge circuit to be determined more accurately than otherwise would be the case.

Two adjustable potentiometers 7 and 8 are connected in parallel one with the other and both in series with equal value resistors 3 and 6 across the supply. The wipers of potentiometers 7 and 8'are connected to the selector switch 9 so as to be connectable individually in dependence upon the setting of selector switch 9 to the source electrode of field effect transistor T1 in the detector circuit. The potentiometers 7 and 8 are linear so as to provide equal voltage variation with equal control knob movements.

The selector switch 9 is mechanically linked to the previously mentioned filter carrier in which are mounted green, blue and red colour filters, the arrangement being such that when the selector switch is set to make with the junction of resistors 4 and 5 the green filter is disposed in front of the photoresistor 1, and when the selector switch is set to make with the wiper of potentiometer 7 or 8 respectively the blue or red colour filter respectively is disposed in front of the photoresistor 1.

The apparatus is powered from a 9 volt battery for simplicity and cheapness. A hold-down on-off switch, which returns automatically to its off position when released, is provided.

FIG. 2 is a perspective view, partly cutaway to show constructional details, of the embodiment of the invention herein described. As shown, the apparatus consists of a housing 20 formed of bent metal sheet with side closures formed of plastics material. The electrical components of the apparatus which in use of the apparatus require adjustment are mounted on the front face of the housing, the same reference numerals are employed in FIG. 2 to indicate these components as are employed in FIG. 1. Other electronic components shown in FIG. 1, such as the detector transistor FET T1 and amplifier transistors T2 and T3 together with bias resistors are mounted on a printed circuit board 21 supported within the apparatus. A push-button switch 22 is mounted in the front face of the apparatus, together with the indicator lamp 10. Reference numeral 23 designates a battery power supply for the apparatus. Selector switch 9 is constituted by two microswitches (one, 24, of which is shown in FIG. 2) controlled by a switch actuating lever 25 which is pivotally mounted about axis 26. The lever 25 is connected at its upper end to a filter carrier 27 which has mounted therein green, blue and red colour filters one, 28, of which can be seen in FIG. 2, and movement of the lever 25 to select any one of the three switch conditions also causes movement of a respective one of the three colour filters in the carrier 27 into the path of light entering the housing 20 through window 29. Guide members 30, only one of which is shown in FIG. 2, are provided for enabling the lens of a photographic colour printer to be registered accurately with the window 29, the guide members 30 being movable in the direction of the arrow in the Figure.

FIG. 3 is an enlarged view of a detail of the apparatus of FIG. 2, showing the mode of fixing of the adjustable resistance 2 and potentiometers 7 and 8 to the front face of the housing 20 and the way in which the scales of these control members are arranged to be movable for the purpose of initially setting up the apparatus. In this Figure, the front face of housing 20 is designated 31, and supported from the front face 31 within the housing 20 by means of posts 32 is a printed circuit board 33(designated 21 in FIG. 2) upon which the electronic components of the apparatus are mounted. A potentiometer 34, which may constitute either of the potentiometers 7 and 8, is mounted on the printed circuit board 33 with its rotatable shaft 35 extending towards and through an opening 36 in the front face 31 of housing 20. A backing plate 37 is supported from the potentiometer 34 by means of a spring 38, and sandwiched between the backing plate 37 and the rear surface of the front face 31 is a rotatable scale disc 39, which has a hole 40 pierced through it to enable the scale disc 39 to be rotated by means of the point ofa ball point pen for example inserted into the hole 40. The scale disc 39 carries on its front face a scale which is visible through the opening 36 in the front face 31 of the housing 20 (see FIG. 2). The strength of the spring 38 is chosen to be sufficient to hold the scale disc 39 fixed once it has been adjusted, and the clearances between the scale disc 39 and the potentiometer shaft 35 and knob are such that rotation of the knob and shaft do not tend to move the scale.

In use of the described apparatus, the adjustable resistance (or rotary switch) 2 is selected to correspond to the green colour component of light transmitted through a negative to be printed and, as previously explained, is calibrated in terms of exposure times. The adjustable potentiometer 7 is selected to correspond to the blue colour component and is calibrated in terms of yellow subtractive filter densities, and the adjustable potentiometer 8 is selected to correspond to the red colour component and is calibrated in terms of cyan subtractive filter densities. As described above, the scales of the three adjustable components 2, 7 and 8 are all movable relative to the control knob of the components for the purpose of initially setting the apparatus by reference to exposure times and filter settings previously determined for a selected negative.

Having set up the apparatus by reference to a selected negative for which optimum printing conditions previously were determined, by directing upon the photoresistor 1 the light from a printer set to operate in accordance with the determined conditions and having the selected negative inserted in its negative holder, then switching the selector switch 9 sequentially to connect first the junction of resistors 4 and and then the wipers of potentiometers 7 and 8 to the source electrode of detector FET Tl each time adjusting the adjustable resistance 2 or the potentiometer 7 or 8 respectively until the lamp just lights, and then setting the scales relative to the control knobs of the components 2, 7 and 8 to correspond to the previously determined exposure time and filter settings for the selected negative, the printing conditions for any other negative are determined as follows.

The colour negative to be analysed is placed in the colour printer, and the light passing through the printer lens is allowed to fall on the CdS photoresistor of the analyser. By means of the selector switch 9, Green, Blue, and Red filters respectively can be brought in front of the photoresistor.

With the filter selector switch 9 firstly in the green filter position, the circuit switch is made in the l position in FIG. 1. This connects the source of the FET T1 to the junction of the resistors 4 and 5, which, together with two 680 ohm resistors 3 and 6 form a voltage divider across the 9 volt supply. The source of T1 is thus held at half the supply voltage, or 4% volts, neglecting the small voltage drop across diode D1.

It will be seen that the CdS photoresistor l is connected in series with the 29-way rotary switch 2. The two in series are connected across the supply voltage, and the junction is connected to the gate of T1. If the rotary switch is in any position where its resistance value is substantially lower than that of the photoresistor, the gate of T1 will be at a potential of less than half the supply voltage, i.e., it will be negative relative to the source, and T1 will not conduct. If the rotary switch 2 is turned in a direction which raises its resistance, the voltage at the gate of Tl'will be raised in a positive direction until eventually Tl begins to conduct. This will occur when the gate voltage of T1 is approximately the same as the source voltage indicating that a position has been reached where the resistance of the rotary switch is equal to that of the photoresistor. When Tl starts to conduct it drives T2, which in turn drives T3, which lights the indicator lamp 10. The high gain given by T2 and T3 results in a sensitive action of the indicator lamp, i.e., at the balance point it switches from completely off to fully on with only a small change of voltage at the gate of T1 typically less than 0.1 volt. In practice, the rotary switch 2 is adjusted to the first position at which the lamp lights up.

So far, with the green filter in place, balance has been obtained by adjustment of the rotary switch. Next, the blue filter is brought into position, which action automatically switches the yellow potentiometer 7 into circuit (switch position 2). NOTE that the rotary switch is not touched again now, but adjustment is made by means of the yellow potentiometer 7.

It will be evident that if the response of the CdS photoresistor to the blue content of the negative is precisely the same as it was to the green content, balance will be obtained when the yellow potentiometer 7 is at its centre point, because at this point the source of T1 will be at half the supply voltage, as was the case when the green measurement was made. If, however, the resistance of the photoresistor is higher (or lower) in the case of the blue filter, the voltage of the gate of T1 will be correspondingly lowered (or raised) and it will be necessary to adjust the source voltage of T1 by means of the yellow potentiometer-7 to find a point where the indicator lamp just lights. The amount it is necessary to offset the source voltage from its original value of half the supply voltage gives an indication of the blue transmission of the negative relative to the green transmission. This yellow potentiometer knob is calibrated to indicate the amount of yellow (minus blue) filter correction needed to print the negative.

Lastly the red filter is brought into place, which automatically switches the cyan potentiometer 8 into circuit (position 3). The latter is adjusted to obtain balance in a manner similar to the yellow potentiometer 7. The

position of this potentiometer gives an indication of the red transmission of the negative relative to green, and it is calibrated directly in terms of the cyan (minus red) correction filter necessary for printing the negative.

Having thus ascertained the exposure time and filter settings necessary for printing the negative, these settings are adjusted on the printer and printing of the negative can then proceed.

The embodiment of the invention illustrated in FIGS. 4,5 and 6 is very similar in regard to its electronic circuitry to the embodiment of FIGS. 1,2 and 3 and differs therefrom mainly in its physical form and particularly in that the photocell of the apparatus is contained in a separate head or lens unit (see FIG. 6) connected to the main analyser unit by means of a coaxial cable and adapted to be coupled to the lens mount of a colour printer or enlarger. As described more fully hereafter, the lens unit of FIG. 6 also contains a movable filter carrier in which green, blue and red colour filters are mounted.

Referring first to FIG. 4, in which components performing the same function as corresponding components in the circuit of FIG. 1 are designated with the same references, it will be seen that the electronic circuitry of the second embodiment differs only slightly from that of the first. As mentioned above, the photocell constituted by photoresistor l is housed in a separate lens unit, coupled to the analyser unit by means of a coaxial cable 20. The lens unit also contains the green, blue and red colour filters and the means for disposing the filters one at a time in front of the photocell, and therefore it is not convenient in this embodiment to arrange that the changing of the filter also switches into circuit the appropriate potentiometer or adjustable resistance. Instead, the selector switch 9 of the previously described embodiment is replaced in this embodiment by three push button switches 9,, 9 and 9 connected as shown. It will be noticed also that the push button switches 9 ,9 and 9 also serve as the ON OFF switch of the analyser and no separate ON OFF switch is provided. The adjustable potentiometers 8 and 7 in the second embodiment have associated therewith resistor-diode networks R R R D D and R R R D D respectively, the purpose of which is to provide electrical linearisation to the adjustable potentiometer scales so that density graduations in all parts of the scales are equally spaced; the mode of operation of these networks is well known and depends upon the change in the biasing of the diodes as the wiper of the potentiometer is moved. The only other difference between the circuit of FIG. 4 and that of FIG. 1 is the addition of diode D, to protect the transistors T,,T and T in case of reversed battery connection.

FIG. 5 shows the second embodiment in front face view. The embodiment consists of a main analyser unit 21 coupled by coaxial cable to a lens unit 22. The main unit 21 consists of a generally rectangular housing the front face of which supports the operating controls of the apparatus, viz. the adjustable resistance 2 serving for exposure time determination, the potentiometers 7 and 8 serving for yellow and cyan filter density determinations, the respective scales associated with the controls 2,7 and and 8, the push button switches 9 ,9 and 9 and the indicator light 10. The fixing of these components to the front face of the analyser unit generally follows the fixing of the corresponding components of the first-described embodiment.

The lens unit 22 is shown in FIG. 6. The unit has affixed to the front face thereof a clamping ring 23 with adjusting acrews 24 which enables the lens unit 22 to be clamped bodily to the lens mount of a photographic colour printer or enlarger. Concentric markings on the front face of the lens unit enable it to be aligned with the lens mount. A circular aperture or window 25 is provided at the centre of the front face of the lens unit and serves to permit light to impinge upon a photocell mounted behind the window 25. A filter carrier 26 is pivoted at one corner so as to be movable to place any selected one of green, blue and red filters 27,28,29 respectively between the window 25 and the photocell so as to filter the light impinging on the photocell.

The second embodiment is used in the same way as the first-described embodiment and operates similarly.

Whilst this invention has been described in relation to the method of colour printing in which the negative to be printed is given a single exposure with colour corrected light, it will be appreciated that the information provided by the apparatus according to the invention (i.e., the requisite colour correction filter densities and exposure time required for printing any negative) is readily convertible into exposure time information for the methods of colour printing where either the negative is initially exposed with white light and the exposures of the different colour components (e.g. green, blue, red) are terminated separately at appropriate times by introduction of appropriate colour filters into the exposing light beam, or alternatively the exposure of the negative to each of the different colour components is carried out separately and successively with appropriate exposure times.

It will be appreciated that the selection of the adjustable resistance 2 to correspond to a green colour component determination, and of the potentiometers 7 and 8 to correspond to blue and red colour component determination is merely a matter of choice, and that the invention is not limited to such an arrangement.

What I claim is:

1. Apparatus for determining the color balance for photographic color negatives which are printed on a color printer having a source of illumination which directs light through the negative and color correction filters to the printing paper, said apparatus comprising:

a two terminal voltage source;

first, second and third primary tricolor filters arranged on a moveable tray;

a photosensitive device whose resistance varies in response to the light impinging upon it, said photosensitive device being located so as to receive part of the light from the source of illumination after it has passed through the negative, the color correction filters and at least one of said primary tricolor filters, one end of said photo sensitive device being connected to the first end of said voltage source;

a logarithmically variable resistance connected between the other end of said photo sensitive device and the second end of said voltage source;

first and second resistances connected in series across said voltage source;

at least one linear potentiometer having its ends connected across said voltage source;

a two input detector means for producing an output whenever the voltages on its inputs are substantially equal, the first input on said detector means being connected to the junction of said photosensitive device and said logarithmically variable resistance; and

switching means for selectively connecting the junction of said first and second resistances and the wiper of said linear potentiometer to the second input of said detector means.

2. Apparatus as claimed in claim 1 wherein the resistance of said logarithmically variable resistance extends over the range of resistances of said photosensitive device.

3. Apparatus as claimed in claim 1 wherein said first and second resistances are equal in value.

4. Apparatus as claimed in claim 1 wherein said photosensitive device is a photoresistor.

5. Apparatus as claimed in claim 1 wherein said detector means is a high gain field-effect transistor input amplifier whose output energizes a lamp.

6. Apparatus as claimed in claim 1 further including a housing separate from said printer for containing said apparatus, said housing having a window in one surface to permit light to enter the housing and impinge upon said photosensitive device.

7. Apparatus as claimed in claim 1 further including a head for containing said photosensitive device, said photosensitive device in said head being connected to said voltage source and said logarithmically variable resistance by electrical leads, said head being coupled to the objective lens of a color printer.

8. Apparatus as claimed in claim- 1 further including a second linear potentiometer having its ends connected in parallel with the said first linear potentiometer, said switching means being adapted to connect the wiper of said second linear potentiometer to the second input of said detector means.

9. Apparatus as claimed in claim 8 further including a pointer knob and moveable scales associated with said logarithmically variable resistance and said linear potentiometers, the scales associated with said logarithmically variable resistance being representative of exposure time and the scales associated with said linear potentiometers, each being representative of filter densities for a different color component.

10. Apparatus as claimed in claim 1 wherein said switching means is operatively connected to the tray of said three tricolor filters so as to place a different one of said tricolor filters between the source of illumination and photosensitive device with each selection of a connection to the second input of said detector means. 

1. Apparatus for determining the color balance for photographic color negatives which are printed on a color printer having a source of illumination which directs light through the negative and color correction filters to the printing paper, said apparatus comprising: a two terminal voltage source; first, second and third primary tricolor filters arranged on a moveable tray; a photosensitive device whose resistance varies in response to the light impinging upon it, said photosensitive device being located so as to receive part of the light from the source of illumination after it has passed through the negative, the color correction filters and at least one of said primary tricolor filters, one end of said photo sensitive device being connected to the first end of said voltage source; a logarithmically variable resistance connected between the other end of said photo sensitive device and the second end of said voltage source; first and second resistances connected in series across said voltage source; at least one linear potentiometer having its ends connected across said voltage source; a two input detector means for producing an output whenever the voltages on its inputs are substantially equal, the first input on said detector means being connected to the junction of said photosensitive device and said logarithmically variable resistance; and switching means for selectively connecting the junction of said first and second resistances and the wiper of said linear potentiometer to the second input of said detector means.
 2. Apparatus as claimed in claim 1 wherein the resistance of said logarithmically variable resistance extends over the range of resistances of said photosensitive device.
 3. Apparatus as claimed in claim 1 wherein said first and second resistances are equal in value.
 4. Apparatus as claimed in claim 1 wherein said photosensitive device is a photoresistor.
 5. Apparatus as claimed in claim 1 wherein said detector means is a high gain field-effect transistor input amplifier whose output energizes a lamp.
 6. Apparatus as claimed in claim 1 further including a housing separate from said printer for containing said apparatus, said housing having a window in one surface to permit light to enter the housing and impinge upon said photosensitive device.
 7. Apparatus as claimed in claim 1 further including a head for containing said photosensitive device, said photosensitive device in said head being connected to said voltage source and said logarithmically variable resistance by electrical leads, said head being coupled to the objective lens of a color printer.
 8. Apparatus as claimed in claim 1 further including a second linear potentiometer having its ends connected in parallel with the said first linear potentiometer, said switching means being adapted to connect the wiper of said second linear potentiometer to the second input of said detector means.
 9. Apparatus as claimed in claim 8 further including a pointer knob and moveable scales associated with said logarithmically variable resistance and said linear potentiometers, the scales associated with said logarithmically variable resistance being representative of exposure time and the scales associated with said linear potentiometers, each being representative of filter densities for a different color component.
 10. Apparatus as claimed in claim 1 wherein said switching means is operatively connected to the tray of said three tricolor filters so as to place a different one of said tricolor filters between the source of illumination and photosensitive device with each selection of a connection to the second input of said detector means. 